C++: Ignore explicit template instantiations

Defined in section 14.7.2 of the standard.

Fixes completion for std::string.

The following explicit instantiation appears in bits/basic_string.tcc:
  extern template class basic_string<char>;

This is wrongfully considered a specialization for a forward declaration
(like `template<> class basic_string<char>` is).

Introduce a new Symbol type for explicit instantiations.

Use-case:
template<class T>
struct Foo { T bar; };

template class Foo<int>;

void func()
{
    Foo<int> foo;
    foo.bar; // bar not highlighted
}

Change-Id: I9e35c8c32f6b78fc87b4f4f1fc903b42cfbd2c2b
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/plugins/cpptools/cppcompletion_test.cpp

@@ -796,6 +796,21 @@ void CppToolsPlugin::test_completion_data()
             << QLatin1String("Data")
             << QLatin1String("dataMember"));
 
+    QTest::newRow("explicit_instantiation") << _(
+            "template<class T>\n"
+            "struct Foo { T bar; };\n"
+            "\n"
+            "template class Foo<int>;\n"
+            "\n"
+            "void func()\n"
+            "{\n"
+            "    Foo<int> foo;\n"
+            "    @\n"
+            "}\n"
+        ) << _("foo.") << (QStringList()
+            << QLatin1String("Foo")
+            << QLatin1String("bar"));
+
     QTest::newRow("use_global_identifier_as_base_class: derived as global and base as global") << _(
             "struct Global\n"
             "{\n"